This invention relates to a method and a device for measuring the pressure inside tubular bodies.
Such tubular bodies are employed typically as nozzles of plastics injection-molding machines, as injection pumps on diesel engines, and as gun barrels. Gases or liquids flow through them, and the most important variable to be measured is usually the internal pressure, either to protect connected equipment against overloading or to monitor or control a process. Often the gases or liquids in question are hot, as is the case with all the above-named applications, presenting special meteorological difficulties.
Pressure is measured more or less directly in all the above-mentioned applications. In ballistics, for example, a pressure transducer is fitted radially into a hole drilled in the gun barrel and passing right through the wall. The transducers have a diaphragm which is in direct contact with the explosion gases. This has the disadvantage that the delicate part of the transducer is exposed to highly corrosive gases, shortening its life. Furthermore, the temperature rise during the explosion causes mechanical stresses in the barrel wall, which may falsify the measurement of the gas pressure. With plastics injection molding machines, on the other hand, it is the pressure in the mold cavity itself firstly, secondly the pressure obtaining in the nozzle, that are of interest for monitoring and controlling the injection process. Here, too, the high temperature of the medium may complicate pressure measuring, as the transducer diaphragm may be damaged, for example, Moreover, when a transducer is fitted into a hole, there is usually a dead volume between the transducer and the tubular body at the diaphragm part, into which the fluid plastic penetrates and solidifies partially. This is equally detrimental to the accuracy of the measurement as it is to the quality of the molding. In all cases, the mechanical stresses due to temperature variations in time and place may falsify the measurement of the pressure inside molds and tubes.
For measuring pressure in fuel injection lines on engines, sensors located in the medium and measuring its pressure directly are employed for the most part. Fitting the pressure measuring instrumentation presents problems where the lines are to be monitored not continuously but only intermittently. The pipework system must then be opened every time to screw the measuring device into the line, which may cause the whole system to leak. Opening and screwing up the systems is also a complicated and time-consuming operation. For this application DE-OS 26 49 358 proposes an improvement in that the pressure is measured not directly but indirectly, a part of the wall with reduced thickness being deformed elastically by the inside pressure in the manner of a diaphragm, this deformation being transmitted to a transducer. But this solution likewise does not take into account the fact that hot media cause mechanical stressing of the walls due to temperature variation, which may falsify the measured result. Furthermore, over wide ranges there is no linear correlation between the deformation of this wall part and the internal pressure to be measured, so that calibration is very laborious.
In EP 0 041 599 a transducer is presented having a thin front plate adapted to the outer shape of the tube instead of a diaphragm. Here again, however, adaptation to the tube form may be laborious. Moreover, the pressure transmission is not as good as on a transducer with diaphragm. On thin tubes the circumference must be enlarged by adding tubular bodies, so that the above-mentioned sleeve-shaped pressure transducer can be adapted to the front plate, whose curvature radius cannot be reduced at will. This makes adaptation more complicated, while the pressure transmission is poorer still.
The transducers mentioned so far assure neither a linear correlation between the gas or liquid pressure (or at least between the difference of inside and outside pressure) and the measured variable over a wide measuring range, nor do they compensate adequately the measuring errors due to temperature gradients and interference voltages in the material. The purpose of the present invention is to overcome these disadvantages. The solution involves detecting the mechanical stresses set up by the pressure at one point at least along the longitudinal axis of the tubular body, in the radial and tangential directions, by means of electromechanical transducers. The output signals of the transducers representing the radial and tangential stresses are processed by subtracting the output signals with like polarity of the transducers and adding them with unlike polarity, so that a measuring signal proportional to the difference between the inside and outside pressures is obtained, which can be fed to a signal evaluation facility for further evaluation.
The device according to the invention is distinguished by at least one pair of electromechanical transducers arranged at one point at least along the longitudinal axis of the tubular body, in order to detect the mechanical stresses set up by the pressure in the radial and tangential directions and to generate analog output signals for the radial and tangential stresses, and a signal processing facility accepting the output signals from the transducers, for creating a measuring signal proportional to the difference between the inside and outside pressures, which may be fed to a signal evaluation facility for further evaluation.
With like polarity of the electromechanical transducers, the signal processing facility may comprise a subtraction circuit, and with unlike polarity an addition circuit.
The electromechanical transducers may be so-called longitudinal measuring pins, i.e., transducers sensitive to stressing in their longitudinal direction, or transverse measuring pins sensitive to stressing in their transverse direction.
A further development of the invention is distinguished by the electromechanical transducers being arranged in a sensing element which can be fitted into the tubular body as a separate intermediate piece.